1. Field of the Invention
The present invention relates to a method for operating a computed tomography (CT) device, of the type having an X-ray source at least the focus of which is, i.e. plane or curved at least two-dimensional displaceable around a system axis, a detector system, formed by a planar/array of detector elements that are arranged in rows that extend substantially transverse to the direction of the system axis and that also are arranged in columns, wherein the detector system picks up X-rays emitted from the X-ray source, and a support device for an examination subject, wherein the focus of the X-ray source is displaced around the system axis for scanning a volume of the examination subject for generating an X-ray shadow image of the examination subject is obtained.
2. Description of the Prior Art
The following method for operating the CT device is normally utilized for carrying out an examination by means of a CT device. An X-ray shadow image (topogram) is generated when the X-ray source does not rotate, with the examination subject being moved on the support device relative to the X-ray source and the detector system is moved in the direction of the system axis. The area of the examination subject to be acquired is defined, in the actual examination, in the direction of the system axis on the basis of the generated X-ray shadow image. The examination subject is positioned by moving the support device to the start point of the examination subject's defined area to be acquired. The actual examination is then conducted, usually in the form of a spiral scan.
This method is associated with a few basic disadvantages. The entire procedure is relatively long, which is undesired for reasons of efficiency and also for medical reasons, particularly with respect to emergency patients. Also, when X-ray shadow images from different projection directions (angles of view) are desired, for example "from front" and "from the side", the patient is moved three times with the patient bed--namely twice for purposes of generating the shadow images and a third time for the spiral scan. There is the danger of the examination results being falsified due to movements of the patient between X-ray shadow images and spiral scan. The generation of one or more X-ray shadow images means an additional radiation exposure for the patient to be examined.
In a CT apparatus having a detector system with only one row of detector elements, these disadvantages can be theoretically avoided by the method known from European Application 0 531 993 and German OS 41 03 588, since merely a spiral scan is carried out and the data that are required for the reconstruction of the X-ray shadow image, from one or more projection directions, are extracted from the thereby-acquired data and are correspondingly processed. The resulting X-ray shadow image, or the resulting X-ray shadow images, keeping pace with tomograms that are reconstructed in parallel with the X-ray shadow image-reconstruction, are shown together on a display unit. However, in practical operation the image quality of the X-ray shadow image is insufficient, particularly when disadvantageous operating parameters are utilized, such as a slice thickness greater than 2 mm and/or a relation of bed advance per rotation to slice thickness (pitch) of greater than 1.